MESSENGER has discovered assemblages of tectonic landforms unlike any previously found on Mercury or elsewhere in the Solar System. The findings are reported in a paper led by Smithsonian scientist Thomas Watters, "Extension and contraction within volcanically buried impact craters and basins on Mercury," published in the December issue of the journal Geology.

The surface of Mercury is covered with deformational landforms that formed by faulting in response to horizontal contraction or shortening as the planet's interior cooled and surface area shrank, causing blocks of crustal material to be pushed together. Contraction from cooling of Mercury's interior has been so dominant that extensional landforms caused by fault formation in response to horizontal stretching and pulling apart of crustal material had not been previously documented outside of the interiors of a few large impact basins.

The MESSENGER spacecraft, in orbit around Mercury since March of last year, has revealed families of extensional troughs, or graben, that are encircled by contractional wrinkle ridges arranged in circular rings. The troughs can form complex patterns varying from the outlines of polygons inside the ridge rings to arcs that parallel the bounding ridges.

"The pattern of winkle ridges and graben resembles the raised edge and cracks in a pie crust," said Watters of the Center for Earth and Planetary Studies at the National Air and Space Museum. The "pie crust" analogy also fits another notable aspect of these collections of tectonic landforms -- their association with "ghost" craters. Ghost craters are impact craters that have been flooded and buried by lava flows. The thin volcanic deposits overlying the rim of a fully buried impact crater serve to concentrate contractional forces, leading to the formation of a ridge ring that reveals the outline of the buried crater.

"The special arrangement of the wrinkle ridges and graben in many of the ghost craters on Mercury is due to a combination of extensional forces from cooling and contraction of unusually thick lava flow units and contractional forces from cooling and contraction of the planet's interior," says Sean Solomon of the Columbia University's Lamont-Doherty Earth Observatory, coauthor and principal investigator of the MESSENGER mission. The eruption and rapid accumulation of very fluid lava flows into thick cooling units on a planet undergoing a high rate of global contraction may be why these systems of tectonic landforms in ghost craters on Mercury have not been seen elsewhere in the Solar System.

As the MESSENGER mission's Deep Space Network (DSN) scheduler, Jessica Call vies for time on the network of large antennas around the globe for scientists and engineers to talk to the spacecraft. "Since each antenna can point only to one spacecraft at a time, schedulers like me negotiate on behalf of the various deep space projects to develop a plan so that each spacecraft can share this valuable resource," she explains. Read more about her critical role on the team here.

MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and entered orbit about Mercury on March 17, 2011 (March 18, 2011 UTC), to begin a yearlong study of its target planet. MESSENGER's extended mission began on March 18, 2012. Dr. Sean C. Solomon, the director of Columbia University's Lamont-Doherty Earth Observatory, leads the mission as Principal Investigator. The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.